Electroluminescent (EL) lamps having different characteristics and electronic circuits used to drive the lamps are shown extensively in the prior art. For example, U.S. Pat. No. 4,527,096, incorporated herein by reference, issued on Jul. 2, 1985 to Kindlmann and assigned to the present assignee, discloses a drive circuit for a timepiece which includes a converter that provides current pulses for charging a capacitive EL lamp to produce light and a switching circuit that switches a predetermined number of successive current pulses at high frequency for progressively charging the EL lamp. The switching circuit, which is also referred to hereinafter as the charging switching means, also reverses the polarity of the electroluminescent lamp at low frequency when the EL lamp discharges at the end of a predetermined number of successive current pulses, and before the EL lamp charges again by current pulses produced by the converter. The switching circuit is connected to and is responsive to signals provided by a timepiece frequency divider circuit (signal generating means), and the converter is connected to a timepiece battery for stepping up the battery voltage to provide for the charging current pulses. The drive circuit of the Kindlmann patent delivers substantially all of the energy provided by the converter to the capacitive EL lamp.
FIG. 1 is a circuit diagram of the drive circuit 10 disclosed in the Kindlmann patent. An inductor 12 has one terminal electrically connected to the anode of a diode 14 and a switch 5 at a common point 13, and a second terminal electrically connected to a battery V.sub.s. The inductor 12-diode 14-switch 5 combination forms the converter 20 that steps up the battery voltage. The converter 20 is electrically connected to the EL lamp 21 through the switching circuit which comprises switches 1, 2, 3 and 4. The gating signals used to open (switch off) and close (switch on) switches 1, 2, 3 and 4 are provided by the timepiece binary frequency divider circuit (not shown) which, as is understood, generally comprises serially connected flip-flops electrically connected to the timepiece crystal controlled oscillator. Switch 5 is also electrically connected to an intermediate stage of the divider circuit (signal generating means). The signal provided to switch 5 is in a predetermined relationship with the gating signals that control switches 1, 2, 3 and 4. The signal provided to switch 5 is at a much higher frequency than the gating signals provided to switches 1, 2, 3 and 4. The current pulses provided by the drive circuit 10 that progressively charge the EL lamp 21 are shown in FIG. 2. Note the discharging and the polarity reversal of the EL lamp after a predetermined number of successive current pulses.
It is well-known in the art that the brightness of an EL lamp is dependent on the applied voltage. See U.S. Pat. No. 4,208,869 issued on Jun. 24, 1980 to Hanaoka, FIG. 15 and accompanying explanation. As the voltage applied to the EL lamp increases, the brightness increases. Conversely, as the applied voltage decreases, so does the brightness. Notwithstanding this common knowledge, none of the drive circuits known by the applicant provides a means for controlling the level of voltage applied to the EL lamp to vary its brightness. Thus, it is one object of the present invention to provide an improved drive circuit that can be controlled to vary the level of voltage applied to an EL lamp.
It is another object of the present invention to provide an improved drive circuit that can be easily controlled by a manual actuating means to vary the level of voltage applied to an EL lamp.
Another object of this invention is to provide an improved drive circuit that converts power efficiently to EL lamps having different lamp characteristics.
Still another object of this invention is to provide an improved drive circuit for driving an EL lamp used for illumination of timepiece displays.